1. Field of the Invention
This invention relates generally to heterocyclic polymers. In particular it relates to novel polybenzimidazoles and to a novel process for the preparation of molecular weight controlled and endcapped polybenzimidazoles.
2. Description of Related Art
Polybenzimidazoles (PBI) are heterocyclic polymers commonly prepared by the condensation reaction of an aromatic bis(o-diamine) with an aromatic diacid or derivative thereof and having a repeat unit of the general structure ##STR1## where Ar is a tetravalent aromatic radical such as 1,2,4,5-tetrasubstituted benzene. Ar may also be a bis(o-diphenylene) having the general structure ##STR2## where Z=nil, O, S, SO.sub.2, O=C, CH.sub.2, or any other appropriate divalent radical. Ar' is a divalent aromatic radical which may be 1,3-phenylene, 1,4-phenylene, 4,4'-biphenylene, 4,4'-oxydiphenylene, 4,4'-sulfonyldiphenylene, or any other appropriate divalent radical.
The synthesis and characterization of PBI has been extensively studied and documented. Reviews on PBI are available. (A. Buckley, D. E. Stuez, and G. A. Serad, Encyl. Poly. Sci. Tech., Vol. 11, 2nd ed., 1988, p. 572; P. E. Cassidy, "Thermally Stable Polymers", Marcel Dekker, Inc., New York, 1980, p. 163.) The first reported synthesis of PBI involved the reaction of aliphatic dicarboxylic acids with aromatic bis(o-diamine)s [K. C. Brinker and I. M. Robinson, U.S. Pat. No. 2,895,948 (Jul. 21, 1959)]. Since then several methods have been utilized in their preparation. Each method will be briefly mentioned with an accompanying reference. The most common synthetic method of PBI involves the melt condensation of aromatic bis(o-diamines) with aromatic diacids or derivatives thereof [H. Vogel and C. S. Marvel, J. Polym. Sci., 50, 511 (1961)]. PBI has also been prepared in polyphosphoric acid [Y. Iwakura, K. Uno, and Y. Imai, J. Polym. Sci., Part A, 2, 2605 (1964)] and in sulfolane or diphenylsulfone [F. L. Hedberg and C. S. Marvel, J. Polym. Sci., Poly. Chem., 12, 1823 (1974)] from aromatic bis(o-diamine)s and aromatic diacids or derivatives thereof.
Another preparative route involves the reaction of aromatic bis(o-diamine)s with the bis(bisulfite adduct)s of dihaldehydes [J. Higgins and C. S. Marvel, J. Polym. Sci., Part A-1, 8, 171 (1970)]. The alkoxide catalyzed reaction of aromatic bis(o-diamine)s with dinitriles is another synthetic route [D. I. Packham, J. D. Davies, and H. M. Paisley, "Polymer, 10, (12), 923 (1969)]. A recent preparative approach involved the nucleophilic displacement of bis(4-fluorophenylbenzimidazole) with alkali metal bisphenates (J. W. Connell in "Adhesives and Sealants in Engineered Materials Handbook", ASM International, 1990, Vol. 3, p. 169).
To obtain polymer with low melt viscosity, the molecular weight of a polymer can be lowered and controlled by using a stoichiometric imbalance of one of the monomers. This is expressed mathematically by a modification of the Crouthers equation for AA-BB type monomers (Polymer Chemistry: An Introduction, 2.sup.nd Ed., M&gt;P. Stevens, Oxford University, New York, 1990, 335-337). EQU DP=(1+r)/(1-r)
where
DP: degree of polymerization PA1 stoichiometric imbalance; 0.5&lt;r&lt;1.0. PA1 M.sub.n =number average molecular weight PA1 MW of repeat unit=molecular weight of polymer repeat unit.
For r=1, there is no stoichiometric imbalance of monomers in the polymerization reaction. The calculated number average molecular weight can then be determined by EQU M.sub.n =(0.5).times.(MW of repeat unit).times.(DP)
where
Endcapping the polymer is achieved by using 2.times.(1-r) of a monofunctional reagent in the polymerization reaction based on monofunctional endcapping technology [D. Progar, T. St. Clair, H. Burks, C. Gautreau, A. Tamaguchi, and M. Ohta, SAMPE J., 52 (January/February 1990)]. An endcapped version of poly[2,2'-(m-phenylene-5,5'-dibenzimidazole)] has recently been reported [M. Jaffe, E. C. Chenvey, W. M. Cooper, M. Glick, and I. Taider, Polym. Prepr., 32(2), 207 (1991)].